Purpose: Large diameter agro-wells are the most valuable water resources for farmers in Mahaweli system H in Sri Lanka to sustain crop production during the dry spells. Deterioration of water quality is harmful to the crop growth. Present study evaluated the variation of irrigation water quality and water levels of agro-wells in Thalawa irrigation block of Mahaweli system H.Research Method: Water samples from 10 agro-wells were collected during October 2016 to July 2017 once in a month and tested samples for irrigation water quality parameters using standard methods. Water levels were recorded for each sampling date.Findings: Most of the water quality parameters tested in almost all wells are within the acceptable range for irrigation except for Residual Sodium Carbonate (RSC). 50 % of the wells are safe and rest 50 % of the wells are within marginal range in terms of RSC. Irrigation water quality index (IWQI) was assessed and it varied from 32.6 to 87.7. Considering the mean IWQI (48.2), water of Thalawa block can be characterized as good. The study also revealed that there was suffi cient water in wells even during the dry spells and it varied with both the variation of rainfall and canal water availability.Research Limitations: Sampling was limited to 10 agro-wells for a period of 10 months. This was avoided by selecting representative wells and months covering both dry and wet spells.Originality/ Value: Overall, results envisaged that there is higher potential to use this water for any crop and soil without detrimental to crops grown.